Computed tomography (CT) imaging is commonly used for imaging anatomical structures, such as lungs. Recently, four-dimensional computed tomography (4DCT) has been attracting much attention. 4DCT provides valuable information in both spatial and temporal scales. In particular, 4DCT may for example be applied to determine an internal volume of a lung tumor.
Nevertheless, breathing irregularities and heart beating at a different frequency may render 4DCT images distorted. Artifacts (discontinuities in the color values of the images not based on the anatomical structure), in particular motion artifacts, frequently occur in 4DCT. Common motion artifacts are duplicate or missing structures in spatially adjacent transversal slices; incorrect re-use of similar projection data during recon of temporally adjacent 3DCT data; artifacts due to violation of the 4DCT data sufficiency condition/low breathing frequencies.
One approach to determine those artifacts is to simply determine color value differences (for color values associated with Hounsfield unit (HU) values) in the 4DCT images. However, generally the determination of 4DCT motion artifacts by this approach suffers from a low sensitivity. In some instances overlapping-type or duplicate-type artifacts may not be detected at all. Furthermore, analysis of secondary data, for example data describing a respiratory signal, may be necessary.
The present invention allows for a more efficient and more precise determination of a position of an artifact in patient image data.
Aspects of the present invention, examples and exemplary steps and their embodiments are disclosed in the following. Different exemplary features of the invention can be combined in accordance with the invention wherever technically expedient and feasible.
The present invention may be applied in all products used to evaluate whether the information derived from the 4DCT is reliable and useable. The present invention is in particular useful for the following products:                pre-planning applications analyzing 4DCTs for ExacTrac® gating/tracking (for example, finding anatomical structures whose motion is well correlated with the tumor motion),        iPlan® RT Dose (radiotherapy dose): 4D treatment planning and also for planning based on the ITV (internal target volume) approach (ITV is the expended CTV (clinical target volume) taking into account the breathing motion, which is usually estimated from 4DCTs).        
The advantages of using the present invention in above products are:                more reliable solutions,                    improved outcome for the patient,                        advantage in the market regarding the ability to evaluate 4DCT quality.        